Liquid water is essential for life, at least here on Earth. Presumably, the same goes for arid Mars. Or that's the assumption that governed NASA's "follow the water" strategy for Mars exploration.

Scientists don't think the liquid was always so scarce. Modern Mars may resemble a barren desert, but very early Mars may have been quite wet, judging from some of the geologic clues left behind. Floods may once have flowed over the planet's surface, rivers may have carved out channels or gullies, and lakes and oceans may have covered large swaths of the planet.

Evidence for this has vastly increased in recent years, with the observations of the Mars Reconnaissance Orbiter, which found thousands of deposits of phyllosilicates at locations around the planet. These claylike minerals arise solely in watery environments -- at temperatures friendly to life -- but were probably laid down in the early days of the solar system, around 4.6 to 3.8 billion years ago. Rovers like Opportunity and Curiosity have revealed that at least some of these lakes maintained salt and acidity levels friendly to life [sources: Rosen; Yeager].

Can't quite picture it? Visit Mono Lake in California, one of the world's oldest lakes at 760,000 years old and an average of 57 feet (17 meters) deep. Now imagine it without water and you'll have the Gusev Crater, a giant basin bisected by a dry riverbed that the Spirit rover searched for evidence of water.

When scientists looked at high-resolution, 3-D images of Mars taken in 2005 and compared them to pictures taken in 1999 of the same area, what they saw excited them: A series of bright, depositary streaks had formed in gullies during the intervening years. These streaks were reminiscent of flash floods that can carve away soil and leave behind new sediments on Earth. A bunch of streaks doesn't sound that monumental, but if water was the recent force behind them, that changes things. (To learn more about the discovery, read "Is there really water on Mars?")

Liquid water may be in short supply, but frozen water isn't. The Phoenix lander investigated the ice in the far north of Mars. The lander's robotic arm dug down into the icy layer for soil samples, which it analyzed with its onboard instruments.

In fact, the lander had three main objectives, all of them water-related: